Real-world assessment of immunogenicity in immunocompromised individuals following SARS-CoV-2 mRNA vaccination: a one-year follow-up of the prospective clinical trial COVAXID

Summary Background Immunocompromised patients have varying responses to SARS-CoV-2 mRNA vaccination. However, there is limited information available from prospective clinical trial cohorts with respect to long-term immunogenicity-related responses in these patient groups following three or four vaccine doses, and in applicable cases infection. Methods In a real-world setting, we assessed the long-term immunogenicity-related responses in patients with primary and secondary immunodeficiencies from the prospective open-label clinical trial COVAXID. The original clinical trial protocol included two vaccine doses given on days 0 and 21, with antibody titres measured at six different timepoints over six months. The study cohort has subsequently been followed for one year with antibody responses evaluated in relation to the third and fourth vaccine dose, and in applicable cases SARS-CoV-2 infection. In total 356/539 patients were included in the extended cohort. Blood samples were analysed for binding antibody titres and neutralisation against the Spike protein for all SARS-CoV-2 variants prevailing during the study period, including Omicron subvariants. SARS-CoV-2 infections that did not require hospital care were recorded through quarterly in-person, or phone-, interviews and assessment of IgG antibody titres against SARS-CoV-2 Nucleocapsid. The original clinical trial was registered in EudraCT (2021-000175-37) and clinicaltrials.gov (NCT04780659). Findings The third vaccine dose significantly increased Spike IgG titres against all the SARS-CoV-2 variants analysed in all immunocompromised patient groups. Similarly, neutralisation also increased against all variants studied, except for Omicron. Omicron-specific neutralisation, however, increased after a fourth dose as well as after three doses and infection in many of the patient subgroups. Noteworthy, however, while many patient groups mounted strong serological responses after three and four vaccine doses, comparably weak responders were found among patient subgroups with specific primary immunodeficiencies and subgroups with immunosuppressive medication. Interpretation The study identifies particularly affected patient groups in terms of development of long-term immunity among a larger group of immunocompromised patients. In particular, the results highlight poor vaccine-elicited neutralising responses towards Omicron subvariants in specific subgroups. The results provide additional knowledge of relevance for future vaccination strategies. Funding The present studies were supported by grants from the 10.13039/501100004359Swedish Research Council, the 10.13039/501100004063Knut and Alice Wallenberg Foundation, Nordstjernan AB, Region Stockholm, and 10.13039/501100004047Karolinska Institutet.

Background Immunocompromised patients have varying responses to SARS-CoV-2 mRNA vaccination. However, there is limited information available from prospective clinical trial cohorts with respect to long-term immunogenicity-related responses in these patient groups following three or four vaccine doses, and in applicable cases infection.
Methods In a real-world setting, we assessed the long-term immunogenicity-related responses in patients with primary and secondary immunodeficiencies from the prospective open-label clinical trial COVAXID. The original clinical trial protocol included two vaccine doses given on days 0 and 21, with antibody titres measured at six different timepoints over six months. The study cohort has subsequently been followed for one year with antibody responses evaluated in relation to the third and fourth vaccine dose, and in applicable cases SARS-CoV-2 infection. In total 356/539 patients were included in the extended cohort. Blood samples were analysed for binding antibody titres and neutralisation against the Spike protein for all SARS-CoV-2 variants prevailing during the study period, including Omicron subvariants. SARS-CoV-2 infections that did not require hospital care were recorded through quarterly in-person, or phone-, interviews and assessment of IgG antibody titres against SARS-CoV-2 Nucleocapsid. The original clinical trial was registered in EudraCT (2021-000175-37) and clinicaltrials.gov (NCT04780659).
Findings The third vaccine dose significantly increased Spike IgG titres against all the SARS-CoV-2 variants analysed in all immunocompromised patient groups. Similarly, neutralisation also increased against all variants studied, except for Omicron. Omicron-specific neutralisation, however, increased after a fourth dose as well as after three

Introduction
Coronavirus disease 2019 (COVID- 19) was declared a pandemic by the World Health Organization (WHO) in March 2020. 1,2 The pandemic subsequently evolved with the emergence of several new SARS-CoV-2 variants-ofconcern (VOC). 3,4 Immunocompromised patient groups were quickly identified as high-risk groups for severe COVID-19 and death. 5 Various platforms were employed to develop vaccines against SARS-CoV-2, including new mRNA-based platforms which demonstrated good safety profiles and high efficacy preventing severe COVID-19 and associated death. [6][7][8] However, pivotal vaccine trials did not include immunocompromised patient groups, creating an unmet need for prospective clinical trials that evaluated safety and immunological responses in these

Research in context
Evidence before this study Immunocompromised patients have increased risk for severe COVID-19 and COVID-19-associated death and respond variably to SARS-CoV-2 mRNA vaccination. Limited information comparing different immunocompromised patient groups with respect to long-term immunogenicity following three and four vaccine doses, particularly in studies that take SARS-CoV-2 infection into consideration, is available. In support of this notion, on March 27, 2023, we searched PubMed for "Clinical Trials" with the following search criteria: ("SARS-CoV-2" OR "COVID-19") AND ("immunocompromised" OR "immunodeficient") AND ("vaccination") AND ("mRNA"). The query returned eight clinical trials. None of the clinical trials identified investigated long-term immunity (>2 months following primary vaccination), the effect of booster doses, and SARS-CoV-2 infection in relation to immunogenicity. Notably, however, other related search criteria returned additional studies related to the present topic.

Added value of this study
The present study, involving 356 study subjects from the COVAXID cohort, followed five major immunocompromised patient groups and respective subgroups as well as healthy controls over one year in a real-world setting. The study subjects were followed since the very first vaccine dose with longitudinal blood samplings and documentation of booster mRNA vaccine doses as well as SARS-CoV-2 infections following an initial two-dose regimen of BNT162b2 mRNA vaccination. In terms of long-term antibody responses to SARS-CoV-2 mRNA vaccination, our data grossly identifies three classes of immunocompromised patients broadly defined by their serological response patterns; 1) strong responders, e.g., patients having undergone HSCT and people living with HIV (PLWH), 2) weak responders, e.g., patients having undergone SOT and treated with MMF, patients with CVID, and patients with CLL treated with ibrutinib, and 3) non-responders, e.g., patients with XLA. The "strong responders" showed responses equivalent to healthy controls over time. Taken together, the study identifies particularly vulnerable patient groups in terms of immunogenicity-related responses among a large group of patients with primary and secondary immunodeficiencies.

Implications of all the available evidence
We here provide a comprehensive, longitudinal assessment of immunogenicity-related responses in a broad range of immunocompromised patient groups. The study allows temporal as well as comparative assessments across many patient groups in a real-world clinical study setting. The findings show that several immunocompromised patient groups need additional booster vaccine doses compared to healthy controls to reach similar levels of immunogenicity. In parallel, subgroups of non-or weak-responders are identified. Articles patient populations. As a result, the COVAXID clinical trial was initiated to address the safety and immunogenicity of the BNT162b2 mRNA vaccine in patients with primary or secondary immunodeficiencies, including those with human immunodeficiency virus (HIV), allogeneic hematopoietic stem cell transplant (HSCT) recipients, solid organ transplant (SOT) recipients, and patients with chronic lymphocytic leukaemia (CLL). 9 Early data from the COVAXID clinical trial showed that the BNT162b2 mRNA vaccine was generally safe, though some adverse immune activation phenomena were observed. Furthermore, varying degrees of antibody (Ab) responses were observed two weeks following the second dose in the different immunocompromised patient groups. 9 While short-and longer-term immunogenicity data from other clinical studies assessing the immunogenicity of SARS-CoV-2 vaccination in immunocompromised patient groups have been reported, 10-17 studies of long-term immunogenicity-related responses among larger sets of immunocompromised patient subgroups are more limited. Here, we report the results of a oneyear assessment of immunogenicity-related responses from patients originally included in the COVAXID prospective open-label clinical trial, in which SARS-CoV-2 specific antibody titres and neutralisation responses were assessed in relation to a third and fourth vaccine dose and, in applicable cases, to SARS-CoV-2 infection (COVID-19).

The COVAXID clinical trial
The prospective open-label clinical trial COVAXID (clinicaltrials.gov; NCT04780659) has previously been described. 9 In short, inclusion criteria included individuals ≥18 years old with no known history of SARS-CoV-2 infection who had either primary or secondary immunodeficiency disorders. Patients were recruited for the study during out-patient visits across various specialties at Karolinska University Hospital, Stockholm, Sweden, with the selection process being impartial to gender. The healthy control group consisted of individuals without an immunocompromised disorder and/or immunomodulatory treatment. The original clinical trial protocol was set to conclude at 6 months after the second vaccine dose. It included two vaccine doses (days 0 and 21) and immunogenicity measurements at six timepoints (days 0, 10, 21, 35 and months 3 and 6). The clinical trial was subsequently extended for a period of up to two years after the second vaccine dose. 356 of 539 (66⋅0%) study participants consented to continued participation in the extended clinical study, which included 2 additional timepoints, 9 and 12 months after second dose. Demographics data such as age and gender, and other medically relevant information was collected via electronic health records and national vaccination registries (Vaccinera), including medications, hospitalisation and vaccination dates. Subgroups were defined based on criteria set at the initiation of the clinical trial. Blood samples and associated clinical data were collected between February 23, 2021 and May 9, 2022. Average follow-up time was 401 days after second dose.

Procedures
Serum and plasma were collected over one year and analysed for anti-SARS-CoV-2 antibody titres and neutralisation as described below. In all cases, the study subjects received monovalent mRNA vaccines according to label (BNT162b2 mRNA, Pfizer-BioNTech and in some cases mRNA-1273, Moderna) (Supplemental Table S1). The third and, in applicable cases, fourth mRNA vaccine dose was scheduled following recommendations by the Public Health Agency of Sweden. In this context, patient groups considered severely immunocompromised were offered a third vaccine dose, in most cases between months 6 and 9, and a subsequent fourth dose between months 9 and 12. In most cases, other patients and controls were offered the third dose between months 9 and 12 (Fig. 1A, Table 1). Clinical study-associated data, including verified SARS-CoV-2 infection, were recorded in an electronic case report form (eCRF). Presence of previous SARS-CoV-2 infection was recorded via in-person or phone interviews associated to each sampling timepoint. Only infections confirmed with PCR and/or rapid antigen test (RAT) was accepted as a verified SARS-CoV-2 infection. Hometesting (RAT) was initiated on the patient's own initiative. PCR tests conducted were verified through manual review of the patients' electronic health records. Additionally, patients were classified as having had a SARS-CoV-2 infection if IgG anti-nucleocapsid antibody titres were >5000 AU/ml (Meso Scale Diagnostics, MSD). Disease severity was assessed during in-person/phone interview and scored using an ordinal scale (1)(2)(3)(4)(5)(6)(7)(8). 18 The categories were defined as follows: 1, not hospitalized and no limitations of activities; 2, not hospitalized, with limitation of activities, home oxygen requirement, or both; 3, hospitalized, not requiring supplemental oxygen and no longer requiring ongoing medical care (used if hospitalization was extended for infection-control or other nonmedical reasons); 4, hospitalized, not requiring supplemental oxygen but re-quiring ongoing medical care (related to Covid-19 or to other medical conditions); 5, hospitalized, requiring any supplemental oxygen; 6, hospitalized, requiring non-invasive ventilation or use of high-flow oxygen devices; 7, hospitalized, receiving invasive mechanical ventilation or extracorporeal membrane oxygenation (ECMO); and 8, death.

Antibody tests
Serum samples were initially tested for pan-Ig, including IgG, to SARS-CoV-2 Wu-Hu.1 (ancestral strain) Statistical tests were performed on paired Spike-RBD titres using Wilcoxon, and Bonferroni correction for multiple comparisons. (E) Seroconversion rates over time in each subgroup as defined by Spike RBD titres ≥0⋅8 AU/ml in the entire COVAXID cohort. Ab titres were quantified using the Roche-Elecsys platform. The star annotation (*) indicates statistical significance at a p-value threshold of 0.05 (or ** for p < 0.01, *** for p < 0.001, **** for p < 0.0001). For sample sizes, please see Table 1. Whiskers for all box plots represents 1.5× IQR.

Ethical considerations
The study, including the extension-study, was approved by the Swedish Ethical Review Board and the Swedish Medical Products Agency (no. 2021-06046-02 and no. 5.1-2021-92151, respectively). Informed consent was obtained from all study participants prior to inclusion in the study.

Role of funding source
The funders did not influence the study design, data collection, data analyses, interpretation, writing of the report, or decision to submit the paper for publication.

Results
In the present study, we followed a large group of study subjects from the COVAXID clinical trial 9 over a period of one year, in a real-world setting, with respect to anti-SARS-CoV-2 immunogenicity related responses ( Fig. 1A and B). A detailed description of the respective study groups and subgroups included in the present clinical study is provided in Table 1. The reconsented COVA-XID study cohort (n = 356) were generally older and had higher proportion of females, whilst antibody titres at day 35 were slightly lower in HC and slightly higher in the CLL group compared to the individuals did not reconsent for the extension of the original COVAXID clinical trial (n = 183) (Supplementary Table S2). Study subjects who consented for the extended study but who were PCR positive (n = 2) or seropositive (n = 16) at the start of the original clinical trial, or did not receive the second dose within the specified time window according to the study protocol (n = 8), were excluded from analyses. Due to the low sample size, study participants in the HC and HIV who had received four vaccine doses (n = 3), and CAR-T treated HSCT patients (n = 2), were also excluded from the analyses. As a starting point, we assessed the temporal dynamics of anti-SARS-CoV-2 Wu-Hu.1 Spike Ig antibody titres over one year using the Elecsys anti-SARS-CoV-2 immunoassay ( Fig. 1C; results from days 0 to 35 incorporated for comparison 9 ). Overall, antibody titres initially peaked at day 35 and then declined until the 6month sampling timepoint in all HC, HIV and HSCT patient groups, with the exceptions of "Early" and "Intermediate" HSCT subgroups (Fig. 1C). An increase in antibody titres was subsequently observed between 6 and 9 months in all study groups (Fig. 1C and D). An additional increase was seen in the PID, allogeneic HSCT, SOT, and CLL groups between 9 and 12 months ( Fig. 1C and D). With respect to specific study subgroups, comparably low antibody titres were observed in the X-linked agammaglobulinemia (XLA) and common variable immunodeficiency (CVID) subgroups over the whole 12 months study period (Supplementary Figure S2). Similarly, in patients having undergone SOT, low antibody titres were observed in both subgroups receiving mycophenolate mofetil (MMF). In patients with CLL, low antibody titres were observed in the group treated with ibrutinib (a Bruton's tyrosine kinase inhibitor) (Fig. 1C). An in-depth comparison with respect to dynamics of antibody titres over time in the respective subgroups is shown in the appendix (Supplementary Figure S2).
Seroconversion (>0.8 AU/ml) at day 35 was the primary endpoint in the original COVAXID clinical trial. 9 In the present follow-up, all study subjects in the healthy control group and nearly all in the HIV and allogeneic HSCT patient groups had seroconverted by the 3-month sampling timepoint and remained seroconverted throughout the study period (Table 1, Fig. 1E). Incomplete seroconversion rate at 12 months (below 95%) was seen in the PID CVID group; in the SOT subgroups with MMF; and in the CLL subgroups with ibrutinib, off ibrutinib, and early-stage untreated CLL (indolent) ( Table 1).
In the present real-world follow-up study, the third and, in applicable cases, fourth mRNA vaccine dose was scheduled following recommendations by the Public Health Agency of Sweden. Hence, the temporal studies described above did not depict responses with respect to number of vaccine doses given and infection during the study period. Therefore, we next stratified into groups having received two, three, or four vaccine doses and, in applicable cases, infection ( Fig. 2A). Focusing initially on non-infected COVID-negative (COVID-) study subjects, higher Spike antibody titres were observed in all study groups having received a third vaccine dose compared to two doses ( Fig. 2A). An additional fourth dose, when provided (not provided to all groups within the present study period), yielded higher Spike antibody titres in the HSCT group ( Fig. 2A). Several of the study subjects were infected (COVID-19) during the study period, in particular following the emergence of the Omicron outbreak ( Fig. 1A and Table 1). Taking infection into consideration, in the COVID-positive (COVID+) study subgroups, two vaccine doses and infection yielded comparable antibody titres to three doses without a recorded infection ( Fig. 2A). Overall, a higher number of vaccine doses and presence of verified infection correlated with higher antibody titres ( Fig. 2A). Importantly, however, comparative analyses among the subgroups revealed large differences. Noteworthy, particularly poor responding subgroups, despite three and four vaccine doses, were identified among CVID (PID), both subgroups with MMF (SOT) and Ibrutinib (CLL) subgroups ( Fig. 2B and C; Supplementary  Figure S3).
This far, all studies described were performed with respect to antibody responses to the ancestral strain (WuHu.1). Next, we sought out to analyse binding antibody responses (titres) to all SARS-CoV-2 subvariants prevailing during the study period. Irrespective of assessed subvariant, overall strikingly similar antibody titres were observed with respect to reactivity towards all SARS-CoV-2 Spike subvariant studied (Fig. 3A,  left), yielding a high degree of correlation in comparative analysis (Fig. 3A, right). This led us to further analyse neutralizing antibody responses to all SARS-CoV-2 subvariants prevailing during the study period. Here, markedly different antibody neutralisation responses were observed between non-Omicron and Omicron variants (Fig. 3B, left), yielding a low correlation coefficient between non-Omicron and Omicron variants (Fig. 3B, right). This in turn prompted us to address neutralizing responses in more detail, focusing on comparative studies between neutralisation against WuHu.1 and the Omicron BA.1 variant. A significant increase of WuHu.1-specific neutralisation was seen following the third vaccine dose in all major study groups (Fig. 3C, upper panel). However, the response from a third dose was more or less absent with respect to Omicron BA.1-specific neutralisation (Fig. 3C, lower  panel). A fourth dose, or the addition of a recorded SARS-CoV-2 infection was associated with increased Omicron BA.1-specific neutralisation in several of the study groups (Fig. 3C, lower panel). When the results were addressed at a subgroup level, the findings above with strikingly poor responses against Omicron BA.1 compared to Wu-Hu.1 was recapitulated in all subgroups following three vaccine doses (compare Fig. 3D  and F; Supplementary Figure S4 Only subgroups with more than five recorded samples are displayed. All Ab titres were quantified using V-PLEX Serology Panels (MSD). Statistical tests performed were Mann-Whitney with Bonferroni correction for multiple comparisons. The star annotation (*) indicates statistical significance at a p-value threshold of 0.05 (or ** for p < 0.01, *** for p < 0.001, **** for p < 0.0001). For sample sizes, please see Table 1. Whiskers for all boxplots represents 1.5× IQR.  Figure S4). In conclusion, these results highlight significantly poor vaccine-elicited neutralising responses towards Omicron subvariants in specific subgroups within the present study cohort.
Finally, while the present study was specifically designed to assess immunogenicity-related parameters in relation to vaccination and infection, we also assessed parameters relating to risk of infection and severe disease. More specifically, we analysed antibody titres in study subjects without a prior record of infection at different sampling timepoints. Study subjects who were later infected, did not have significantly lower antibody titres compared to those who were not infected (Fig. 4A). Furthermore, we also correlated antibody titres with COVID-19 severity among those who were infected. In this context, six of the 75 study subjects who had PCR/RAT verified COVID-19 were hospitalized (COVID-19 severity ≥3). Five of the six study subjects were infected following the Omicron outbreak. A tendency towards lower antibody titres in higher severity scores was observed ( Fig. 4B and C). Since the current study was underpowered for, and not intended to investigate the prospective risk of infection and disease severity, we choose not to make any strong conclusions based on these findings.

Discussion
Longitudinal clinical studies addressing immunity in large groups of immunocompromised patients in a realworld setting, taking vaccination as well as infection into consideration, are scarce. Such studies serve the purpose of addressing the dynamics (temporal) of antibody binding titres as well as neutralizing titres in a comparative fashion among different immunocompromised patient groups and subgroups. Here, we report one-year results from such a study, based on a study cohort derived from the original open-label, prospective, clinical trial COVAXID. The results reveal that most of the immunocompromised patient subgroups who responded poorly following the initial two doses of the BNT162b2 mRNA vaccine develop significant binding antibody titres against both the Wu-Hu.1 ancestral strain and Omicron variants following subsequent booster vaccine doses, and in applicable cases SARS-CoV-2 infection. However, while several of the study groups developed neutralizing antibody responses towards the ancestral strain, markedly lower responses were observed towards Omicron subvariants. Notably, Omicron-specific neutralisation increased after a fourth dose as well as after three doses and one infection in many of the patient groups.
In terms of long-term antibody responses to SARS-CoV-2 mRNA vaccination, our data grossly identifies three classes of immunocompromised patients broadly defined by their serological response patterns; 1) strong responders, e.g., patients having undergone HSCT and PLWH, 2) poor responders, e.g., patients having undergone SOT and treated with MMF, patients with CVID, and patients with CLL treated with ibrutinib, and 3) non-responders, e.g., patients with XLA. The "strong responders" showed responses equivalent to healthy controls over time. Although patients having undergone HSCT are still recommended to follow certain healthrelated precautions, we did not observe a compromised long-term antibody response following repeated vaccinations. The latter, most likely due to their reconstituted immune system over time. The "poor responders" comprise of a diverse group of patients. Many of the poorest responders were patients undergoing concurrent immunosuppressive treatment during vaccination. Some of these patient groups may benefit from repeated and/or more frequent vaccinations. The "non-responders" are incapable of responding with antibody responses due do their underlying disease. Importantly, some of these patients have been shown to generate strong T-cell mediated response following vaccination. 19,20 Notably, study participants included in the present real-world study were subject to necessary clinical interventions due to their specific medical conditions. The latter has included therapeutic interventions with immunoglobulin replacement therapy (IGRT), affecting antibody titres in some patient groups (Table 1). In this respect, newer batches of IGRTs contain antibodies against the SARS-CoV-2 Spike and, to a lower degree, Nucleocapsid antigens. 21,22 These antibodies likely contributed to observed antibody titres, and corresponding seroconversion, over time in some of these groups, e.g., the CVID and XLA groups ( Fig. 1 and Table 1). Likewise, a few patients from the study groups were treated with anti-SARS-CoV-2 monoclonal antibodies ( Table 1). The number of infections in the different groups after vaccination did not differ significantly. However, the number of serious covid-infections were very few across the groups, underscoring the fact that vaccination was overall efficient in preventing severe disease, whereas the protection against mild disease with omicron variants was rather poor. However, surrogate virus neutralisation kit (Meso Scale Diagnostics, MSD). (D-G) Neutralisation of Spike Wu-Hu.1 and Spike BA.1 following three or four vaccine doses in each patient subgroup, ordered by their respective mean neutralizing capacity. Only subgroups with more than five recorded samples are displayed. Statistical tests performed were Mann-Whitney with Bonferroni correction for multiple comparisons. The star annotation (*) indicates statistical significance at a p-value threshold of 0.05 (or ** for p < 0.01, *** for p < 0.001, **** for p < 0.0001). For sample sizes, please see Table 1. Whiskers for all boxplots represents 1.5× IQR. risk for infection also correlates with exposure. In this context many of the present patient groups take precautionary measure due to their underlying disease.
A strength of the present study is that data were analysed from a well-defined patient cohort based on the original clinical trial COVAXID, with set inclusion and exclusion criteria as well as independent study monitoring of data, all of which contributed to data quality. Another strength of the study is that patient groups were followed from a timepoint when they were SARS-CoV-2and vaccination-naïve through the emergence of updated national vaccination schedule regimens and in parallel risk for exposure to multiple new SARS-CoV-2 variants, all of which together provided the possibility for a complete and comprehensive longitudinal serological assessment of the study cohort in a real-world setting. The number and frequency of study participant samplings, as well as the ability to directly compare results across subgroups and corresponding different immunodeficiency disorders, provided additional strength to the study. With respect to limitations, as mentioned the individual patients had access to the third and fourth vaccine doses based on priority as recommended by the Public Health Agency of Sweden, i.e., not as part of a pre-defined clinical schedule. Furthermore, while most patients received the mRNA BNT162b2 (Pfizer/BioNTech) vaccine, some received the mRNA1273 (Moderna) vaccine for their third and/or fourth doses (Supplementary Table S3). In this respect, studies have observed slightly better serological responses with the mRNA1273 vaccine than with the BNT162b2 mRNA vaccine. 23 With respect to COVID-19, the diagnosis annotated to patients in the study groups which were PCR or rapid antigen test (RAT) positive, and/or had presence of anti-nucleocapsid antibodies (>5000 AU/ml). The sensitivity and specificity of antinucleocapsid antibodies for PCR/RAT verified COVID-19 in our cohort was 0.44 (0.36, 0.52) and 0.95 (0.94, 0.96), respectively (Supplementary Figure S5), which could predispose for false-negative COVID-19 annotations. Furthermore, anti-nucleocapsid antibody titres may occur in patients who have received IGRT which could predispose for false-positive COVID-19 annotations. An additional limitation of our study pertains to unaddressed confounding factors potentially affecting measured antibody titres. Specifically, variables such as the age, gender and timing, type, and dosage of immunosuppressive treatment administered during the follow-up period were not adjusted for in the antibody titre measurements due to the limited sizes of the subgroups in the current study.
In conclusion, the present results provide a detailed comparative assessment of immunogenicity in several groups of immunocompromised patients following the third and fourth doses of mRNA vaccine as well as the result of SARS-CoV-2 infection in a real-world setting.